dynamic responses of mammals to the eruption of volcan

Mastozoología Neotropical; 1(2): 113-122
SAREM, 1994
ISSN 0327-9383
DYNAMIC RESPONSES OF MAMMALS TO THE
ERUPTION OF VOLCAN HUDSON
Sergio L. Saba and Daniel A. de Lamo
Departamento Biología General. Facultad de Ciencias Naturales Sede Puerto Madryn. Universidad
Nacional de la Patagonia S.J.B. y Centro Nacional Patagónico (CENPAT-CONICET). Bvrd. Alte.
Brown s/n, 9120 Puerto Madryn, Chubut, ARGENTINA
ABSTRACT: Ash coming from the eruption of Volcán Hudson significantly affected the
densities of wild populations of mammals inhabiting an important fraction of Argentine
Patagonia. The convergence of several climatic and biological factors generated a fast
recovery of some populations. A significant increase in some rodent populations (mainly
Eligmodontia and Phyllotis) one year alter the eruption was observed. This response may
be attributed to the abundance of grass, reduced competition with livestock and diminished
natural enemy densities, particularly predatory birds. The recovery of hare population was
faster in the farming area of Los Antiguos, but slow in the plateau area. Large mammals
(e.g. guanaco) are recolonizing the impacted land. The situation at present could be a
step in the ecosystem dynamics where diversity and distribution of some wild mammalian
species tends to be as was before the eruption.
RESUMEN: Respuesta dinámica de los mamíferos a la erupción del volcán Hudson.
Las poblaciones de mamíferos silvestres de una parte importante de la Patagonia fueron
afectadas significativamente por las cenizas provenientes de la última erupción del volcán
Hudson. La convergencia de factores climáticos y biológicos generaron la posibilidad de
una rápida recuperación en algunas de estas poblaciones. Se detectó un incremento
significativo en poblaciones de algunos roedores (principalmente Eligmodontia y Phyllotis)
un año después de esa erupción. Este tipo de respuesta puede ser atribuida a la abundancia de gramíneas, disminución de la competencia con el ganado doméstico y disminución en la densidad de depredadores naturales, particularmente aves de presa. Respecto a la liebre europea, su recuperación fue más rápida en la zona de chacras de la
localidad de Los Antiguos que en la estepa. Los guanacos están recolonizando el área
impactada. Al presente, la situación parece constituir un paso en la dinámica del ecosistema
en el que la diversidad y distribución de mamíferos silvestres tiende a asemejarse a su
estado previo a la erupción.
Key words: mammals, environmental impact, volcano eruption, Patagonia
INTRODUCTION
The Volcán Hudson (45° 90' S; 72° 96'
W) suddenly erupted on August 8,
1991, generating a dense cloud of
pumice and ash of basaltic origin that
crossed Chubut Province in an ENE
direction. A week later, a second larger
and prolongued eruption of trachyandesitic
composition took place. The tephra
ejected from the volcano between
Recibido 28 Febrero 1994. Aceptado 30 Mayo 1994.
August 12 and 15, 1991, was deposited on a triangular area of Patagonia,
close to the Chilean Pacific coast,
reaching Puerto Deseado and Puerto
San Julián on the Atlantic coast of
Santa Cruz Province in Argentina.
About 100,000 Km2 were covered by
ashes (crystals and vitreous materials)
fluctuating on average between 1 and
10cm thick, ejecting between 2-3 km3
114
of material (Corbella et al.,1991).
The eruption of 1991 seemed to be
stronger than the previous in 1971,
which had produced a considerable
rain of ashes (Dust Veil Index, DVI
=250; Lamb, 1972).
The area covered by ash was subject
to severe desertification before the
eruption. Several sheep ranches were
abandoned in the central and northern
portion of Santa Cruz Province, attributable to land degradation (by overgrazing and wood cutting) and capital
loses due to the low international
prices of wool (PPCD INTA, 1993).
Information about the effects of the
eruption on wildlife is still scarce and
fragmentary. Corbella et al. (1991)
point out that a few days after the
eruption, some small birds, rheas
(Pterocnemia pennata) and guanacos
(Lama guanicoe) were affected by
some son of blindness.
In september 1991 a survey aiming
to evaluate the effects of ashes on the
natural ecosystem and the effects on
human settlements in the Province of
Santa Cruz was conducted. In this report we present the results of the study
on the dynamic response of wild mammals after the eruption.
MATERIALS AND METHODS
The area affected by pumice and ash
from the Volcan Hudson in Argentina,
belongs phytogeographically to the
Provincia Patagónica, Distrito Occidental y Central (Soriano, 1956). The mean
annual rain (m.a.r.) is about 150 mm.
The temperature ranges between 1-3°
C (July), and 14-17° C (January). Los
Antiguos is in a relatively rainy location (254 mm m.a.r) and the dominant
winds are from the W (De Fina et al.,
1968). Three field trips, designed to
S.L. Saba and D.A. de Lamo
maximize information retrieval were
conducted. That is 4, 18, and 28 months
after the eruption (m.a.e.). The first
was in December 10-13, 1991. The
second, between February 25-28,
1993, and the third one between November 29 and December 3, 1993.
These trips will be called Survey I, II
and III (SI; Sil; SIII), respectively.
Several rural dwellers were interviewed
on SI, in order to obtain information
about the effects of ashes on their livestock and on some conspicuous wild
species. This information served as a
base on which to place a sampling
design aiming to evaluate the ecosystem changes after the eruption.
Sampling was based on: animal activity determined by strip transects (in
SI, SII and SIII), nocturnal censuses
of hares (Lepus europaeus) by line
transects (in SI; SII and SIII) and by
live trapping of small rodents (in SII
and SIII). Four sampling sites were
defined. Site 1 corresponds to a control area, not affected by the eruption.
The other three sites were located in
a highly impacted area (Site 2) and in
moderately affected areas, respectively
(Sites 3 and 4, Fig. 1). Four strip
transects (50 m long; 2 m wide) were
analyzed in each site. Signs of animal
activity, such as feces, tracks, burrows
or direct evidences were recorded
every meter.
Site 1 corresponds to a plateau. It is
a steppe with bushes and grasses dominated by Mulinum spinosum, Stipa
speciosa and Poa lanuginosa, located
to 20 km of Río Mayo. The vegetation
cover is about 40%. Phytogeographically, it belongs to the Ocidental District. Since no evidence of ashes were
found in this area, it has been considered as a control site for operational
purposes.
115
IMPACT OF THE VO HUDSON ERUPTION
Site 2 is an alluvial plain of the
Jeinimeni River, characterized by
Mulinum spinosum, Adesmia campestris, Senecio filaginoides and Stipa
humilis. The vegetation cover is about
25%. The deposition of ash fluctuated
between 19 cm on vegetation mounds,
and 27 cm between the mounds. This
site showed the highest impact of those
surveyed in this study.
Fifty four km south of Perito
Moreno, north to Ea. La Vizcaína,
there is a glacier valley corresponding
to Site 3. Phytogeographically it belongs to the Central District, subdistrict
Santacrucence. It is an East oriented
hillside with gentie slope, characterized by the presence of Nassauvia
glomerulosa, Verbena sp., Stipa
chubutensis, Stipa ameghinoi and Poa
duseni, with a vegetation cover of 25%.
Ash concentrated on mounds (8 cm
high) and one layer of 3 cm thick was
measured on the intermounds.
Site 4 is located 80 km from Bajo
Caracoles, on Ea. La Frisia. The landscape is characterized by gorges descending from a volcanic plateaus of
Jurassic age (Barrio, 1993). Phytogeographically it belongs to the Central
Fig.1: Map of strip transect Sites (1-4) and areas A and B where rodent live traps were set after the
eruption of the Hudson volcano. 1: Site 1 (control area); 2: Site 2 (high impacted area), between
the towns of Los Antiguos (Arg.) and Chile Chico (Ch.); 3: Site 3 (medium impacted area); 4: Site
4 (medium impacted area). For a description of each area, see text.
116
District, subdistrict S antacrucence
whose dominant species is Verbena
tridens. The vegetation cover is about
60%. The deposition of ash fluctuated
between 11 cm on mounds and 4 cm
deep on the vegetation intermounds.
Line transect counts of hares were
performed in a pick-up truck travelling for 3 consecutive nights, over 2
round trip transects (i.e. 4 line
transects). Two of them (4.8 km long
each) were located around the farms,
in the basin of the valley River Los
Antiguos, a high productivity area
under irrigation. The other two line
transects (10 km long each) were
checked close to Los Antiguos, a semiarid zone with low primary productivity used for extensive sheep breeding. Every hare observed was recorded.
Lines of 100 rodent live traps were
set in two study areas near Los
Antiguos (areas A and B, Fig. 1),
covered by ashes. Area A is a border
of a glacier valley, located on a plateau 7 km E from Los Antiguos.
Vegetation is characterized as a bushy
steppe with Colliguaya intergerrima,
Mulinum spinosum and Stipa humillis
with a vegetation cover of about 45%,
showing a strong anthropic impact
associated with overgrazing and fire.
Area B is in a fluvio-glacier plain
located 2.3 km from Los Antiguos, on
the road to River Zeballos. Vegetation is a bushy steppe with Colliguaya
intergerrima, Mulinum spinosum and
Stipa humilis with about 50% of vegetation cover. Both areas, phytogeographically belong to the Central District, subdistrict Santacrucence.
Sherman traps (27 x 9 x 7.5 cm)
were used with standard bait. One
hundred traps were set for two consecutive nights in area A and one night
S.L. Saba and D.A. de Lamo
in area B during SII. During SIII they
were activated for 2 consecutive nights
in each study area. The animals captured were sexed, measured and their
skulls and pelts were prepared.
RESULTS
As was informed by ranchers, the main
initial consequence of the volcano eruption was a reduction of livestock numbers either by selling them or due to
mortality attributable to lack of resources. On the other hand, ranchers
informed that birds and some conspicuous mammals, notably hares and guanacos emigrated from the affected areas. It coincides with the fact that we
found few corpses in the explored area.
From our periodical sampling, we
obtained the following results:
a) Animal activity signs detected by
strip transects in Sites 1-4 respectively,
are presented in Table 1. In the control area (Site 1), an important animal
activity was evident during SI, mainly
guanacos, hares, and sheeps. The activity was reduced in SII and SIII, and
it may correspond to a recolonization
of the areas affected by the ashfall.
Table 1: Frequency of animal activity signs obtained from
strip transects during surveys I, II and III and Site 1
(control area), Site 2 (high affected area) and Site 3 and
4 (medium affected areas).
Sheep
Species
1 2 3 4
Sites
Surveys
88 0 2 2
I
II
21 O 12 1
III
6 0 0 0
Guanaco
2 3 4
11 0 1 2
6 0 0 2
1 0 0 6
1
Fox
2 3 4
1 0 0 0
1
0 0
4 0 2 0
1 2 8 4
0 0 0 2
0 1 0 0
Species
Armadillo
Sites
Surveys
1 2 3 4
II
III
1
0
1
Hares
2 3 4
40 11 9 2
0 75 33 12
2 39 11 22
1
Rodents
2 3 4
1 0 0 2
O 14 25 11
1 1 7 6
117
IMPACT OF THE VO HUDSON ERUPTION
In Site 2, only few signs of hare
activity were recorded during SI. Since
it is situated in an international border
area, sheep breeding is restricted. During SII, several signs of animal activity were found, particularly from hares
and rodents, which diminish during SIII
(see Table 1).
In Site 3, during SI, few animal signs
were found. The scarce activity was
mainly from hare and in a lower degree from sheep and guanaco. Animal
activity increased 18 months after the
eruption (SII), particularly with sign&
from hares and rodents (burrows, feces and tracks). Slight signs of activity
are from Edentates (armadillos, probably Zaedyus pichiy). This last element
from the fauna increases in SIII, reaching a similar level to those found for
hares and rodents, which showed an
evident decrease compared to SII.
In Site 4 the pattern is similar to Site
3 with respect to rodents and armadillos; however, hare and guanaco
activity increased from SI to SII and
to SIII. General wild animal activity
had similar magnitudes in Sites 1 and
4, but the trends were inverse (54, 10
and 5 animal activity signs, total 69 in
Site 1 vs. 6, 27, 38, total 71 in Site 4;
SI, SII and SIII respectively). Activity
was higher in Sites 2 and 3 with similar trends through time (11, 89, 43,
total 143 and 10, 60, 26, total 96, for
Sites 2 and 3, respectively). A peak of
high animal activity was detected in
the Sites affected by ashfall (2, 3 and
4) 18 m.a.e. (27, 176, and 107 animal
activity signs in SI, SII and SIII respectively).
b) Nocturnal hare censuses by line
transects: during SI we did not record
any observations of hares. Relative
abundance of hares, considered as the
sum of individuals observed every
night over the distante driven, begins
to be important in February 1993 (mean
=0.183 ind/km, SD =0.288), increasing in December of the same year
(mean =0.356 ind/km, SD =0.269). The
difference between both observations
is not significant (T test, p =0.123).
Separating the observations by landscape unit (two line transects in the
farms and two in the plateau), it may
be seen that
the highest relative density of hares
was reached 18 m.a.e. in the farming
area, being low in the plateau. Similar
results were obtained 28 m.a.e. for both
types of environment (Table 2). Relative density of hares calculated for the
plateau 28 m.a.e. was significantly
higher (T test, p =0.018) than that
calculated for the same environment
10 months earlier.
c) Live capture of Rodents: during
SII, from a total of 300 trap nigths
activated in both aneas, 32 were sprung
by intensive wit,c1Then, 173 trap
nigths were active at study area A (2
consecutive nights) and 95 at study area
B (1 night). A total of 26 animals were
captured in area A (trap success
15.03%) and 9 animals in area B (trap
success 9.47%). The capture composition is presented in Table 3.
Table 2: Hare (Lepus europaeus) relative abundance
(ind/km) by landscape unit, obtained from SII and SIII
(during SI no count were recorded).
Farm landscape
Plateau
Survey II
Survey III
Mean: 0.28
S.D.: 0.39
Mean: 0.31
S.D.: 0.315
Mean: 0.03
S.D.: 0.05
Mean: 0.38
S.D.: 0.30
S.L. Saba and D.A. de Lamo
118
During SIII, no animals were caught
at area B (2 consecutive nights, 142
effective traps nights) and only 3 animals were captured at area A (2 consecutive nights, 183 effective trap
nights, 1.64% trap success). The species are Reithrodon auritus (young
male) and Abrothrix xanthorhinus (1
young male; 1 adult male).
A detailed description of captures
from SII and SIII are presented in Fig.
2, as capture success (%) per species.
DISCUSSION
The high faunistic activity measured
by strip transects in the control area,
agrees with the information obtained
by ranchers from the region. Wildlife
migrated from the impacted area after
the eruption. As time passed, range
condition of the affected areas tended
to be like that of the non affected ones,
and animal activity on the control area
decreased.
Ash from the volcano may have affected large (guanacos), medium size
(hares, armadillos) and small mammals
(rodents) as well. However, 18 months
after the eruption, a strong increase in
the relative abundance of those mam-
mals was observed. In the high impacted zone (Site 2) and in one of those
of medium impact (Site 3), the activity of the mammalian fauna was reduced more 28 m.a.e. than the activity
measured 18 m.a.e.. In the other area
with medium impact (Site 4), guanaco
and hare activity was still increasing
while rodents showed a steady state.
Armadillos increased their presence
continuously between 4 and 28 months
after the eruption.
Generally speaking, the abundance of
hares decreased in the summer compared to the winter. The relative abundances calculated in Surveys II and III
are still lower than calculations for the
Precordillera in Río Negro Province
(5.1 ind/km July; 3.6 in August; 3.8 in
September and 2.6 in October; Amaya,
1981). The relative density of hares
seems to be stable in the farming zone
(chacras) 18 and 28 months after the
eruption, increasing in the platean during the same period of time.
Amaya et al. (1979), in a study on
1,293 hares, described that the reproductive season for this animal in
Patagonia extends from late August to
late January. In other words, the depo-
Table 3: Rodents captured during SII, with description of sex and reproductive status.
Eligmodontia morgani
Phyllotis xanthopygus
Ctenomys sp.(3)
Reithrodon auritus
Mus musculus
Reproductive stage (1)
Adults
Juveniles
males
females
females
males
3(2)
2
2
16
2
1
3
(1) Determined by reproductive tract development.
(2) 1 lactating, 1 pregnant carrying 6 fetuses (3/3, right and left horn respectively)
and 1 pregnant carrying 7 fetuses (5/2).
(3) Probably C. coyhaiquensis, new species in D.A. Kelt and M.H. Gallardo (1994).
2
1
1
2
119
IMPACT OF THE VO HUDSON ERUPTION
sition of ash in 1991 must have affected the repoductive cycle for this
species. Burial of the ash by plowing
and cleaning activity in the area around
Los Antiguos may have reduced the
impact of the eruption on the hare
population regularly inhabiting the
farming zone. This fact could have generated a faster re-colonization of the
farms compared to the plateau areas.
In this last environment, relative densities of this Lagomorph measured 28
m.a.e. were similar to those calculated
for the same species in the farming
zone 18 and 28 m.a.e..
Live capture of small mammals during Survey II, mainly Eligmodontia (n
=23) and Phyllotis (n =9) is significantly higher for this son of environment (see Pearson, 1994). The populational increment of these seed and
green eaters could be explained by the
higher supply of herbs, short gramineous and ephemeral plants. Revegetation in the impacted zone by regrowth,
tillering, elongation of meristems aboye
the ash level, and recruitment of new
plants has been reponed (Oliva et al.,
1993a). On rodent capture sites, 18
months after eruption, a significant cover
of Poa lanuginosa was observed
(Bertolami, com. pers.) that could be an
indicator of better range condition.
On the other hand, in survey I and II
Sergio L. Saba and Daniel A. de Lamo
Dynamic responses of mammals to the eruption of Volcan Hudson
14
E
G
12
10
o
E
8
6
P
Q. 4
2
R il
o
A
B
SII
TC
A
SIII
Fig. 2: Rodent live capture success (in percentage) for study areas A and B and total capture (TC) during Survey II (SII:
February 1993) and area A in Survey III (SIII: December 1993). In study area B no captures were recorded
in Survey III. E: Eligmodontia morgani, P: Phyllotis xanthopygus, M: Mus musculus, C: Ctenomys coyhaiquensis,
R: Reithrodon auritus, A: Abrothrix xanthorhinus.
120
no predatory birds were recorded at
the capture sites, in agreement with
Serret' s (1992) report for the highly
impacted areas. During survey III, direct observations of cinereous harrier
(Circus cinereus), red-backed hawk
(Buten polyosoma), american kestrel
(Falto sparverius), black-chested buzzard eagle (Geranoaetus melanoleucus)
and burrowing owl (Athene
cunicularia) and great horned owl
(Bubo virginianus) regurgitated pellets were registered (Saba et al., in
prep.). In our study, relative abundante
of another rodent predator, the grey
fox (Pseudalopex griseus) was scarce.
We consider that our sampling design
was small to detect patterns of activity
of this territorial animal, which usually has a wide honre range and low
densities. Any interpretation of the
few signs found may be confusing.
We understand that a series of factors such as food availability, absence
of predators and reduction of livestock may explain the surprising populational peak observed in Eligmodontia
and Phyllotis.
Twenty eight months after the eruption, the environmental conditions between impacted and not impacted areas
seemed to be similar. Capture success
of different rodent species with regular densities for the summertime (see
Pearson, 1994) were obtained in this
study in SIII.
CONCLUSIONS
Information about the response of wild
mammals to volcanic catrastophic
events is scarce. The eruption of Volcán
Paricutín (México) in 1943 and Mount
St. Helens (USA) in 1980 are examples
of these studies. The eruption of Volcán
Hudson in 1991 affected an arid to
S.L. Saba and D.A. de Lamo
semi-arid environment in southern
Patago-nia and was a unique opportunity to follow the response of mammals in this sort of ecosystem. In spite
of the lack of information about the
condition before the eruption, we
detected a sequence of events that allowed us to give a description and to
formulate an interpretation of the facts.
The eruption of Volcán Hudson
generated an acute impact over an area
that had been previously altered by a
severe process of degradation of
anthropic origin. The mammalian fauna
show signs of fast recovery from the
impact of the eruption.
At first, a reduction of wild mammal
populations and a drastic reduction in
the number of sheep was the result.
This situation, plus a higher precipitation regime during 1992 (235.01 mm
vs. a mean for 1951-1960 =116 mm,
Perito Moreno, Servicio Meteorológico
Nacional) on a layer of ash acted as
an adequate substrate for the establishment of annuals and rhizomatous
grasses (Oliva et al., 1993b). The conjunction of biotic and abiotic factors
may have favored a pronounced vegetation recovery. Then, in a second
stage, a fast population growth of
mammals (particularly rodents), took
place. The demographic increase and
the amazing species diversity of small
mammals could have been facilitated
by the lower degree of competition
with domestic herbivores and a time
lag in the recovery of natural predators, particularly birds.
The re-entrance of natural predators
and the lower availability of forage
caused by inadequate rain during 1993
(101.9 mm, Perito Moreno, Servicio
Meteorológico Nacional), could have
produced conditions similar to those
122
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